In the resin molding step for resin molded-type semiconductor devices, resin molding is repeated several times, so that resin burrs and dirt, such as oil or dust, are accumulated in the inside of the mold, in which a molding resin is filled, or in the cavities, runners and air vents of a pair of top and bottom halves constituting the mold and also in a cull block and its neighbor.
Such dirt adversely influences the quality of moldings and leads to a lowering of releasability when a product is removed from the mold. Accordingly, it is necessary for a worker to clean the mold at intervals of given shots.
However, the cleaning of a mold by a worker is a manual work, which requires much time. Thus, there is a demand for a technique wherein a mold can be cleaned within a short time.
For a technique meeting the requirement, a method has been carried out, as stated in Japanese Laid-open Patent Application No. Hei 1(1989)-95010, wherein a lead frame not mounting a semiconductor chip (hereinafter referred to as dummy lead frame) is clamped between the main surfaces (faces) of a mold, and a cleaning resin formed of a melamine resin or the like is charged into the mold and hardened to cause dirt to be deposited on the cleaning resin surface, thereby removing the dirt along with the cleaning resin.
Alternatively, there is known a method of directly pouring a cleaning resin into the cavity at a high pressure or normal pressure without use of any dummy lead frame or the like.
However, according to the first-mentioned technique, an expensive dummy lead frame has to be used for cleaning, and not only this is poor in economy, but also a dummy lead frame of a specific shape matched with a mold has to be set in position and clamped, so that precise positioning between the mold and the dummy lead frame becomes necessary. Moreover, with a molded resin for cleaning, the resin formed at cull and runner portions is separated out of the lead frame and the removal of this separated resin from the mold requires much time, thus being poor in working property. In addition, the separated cull and runner may be caught in a sliding portion of a molding apparatus, with the possibility of causing mechanical breakdown.
In order to solve such problems as stated hereinabove, a technique set out below has been proposed.
As described in Japanese Laid-open Patent Application No. Hei 6(1994)-254866, the technique comprises the steps of clamping a sheet-shaped cleaning member made of cotton cloth (non-woven fabric), in which a cleaning resin can be impregnated and permeated, between opened mold halves, and filling a cleaning resin in a molten state in the cavity of the closed mold.
As described in the known instance, when a cleaning resin in a fluid form is charged in such a state that the sheet, in which a cleaning resin and chemicals are able to be impregnated therein and permeated therethrough, is placed between the main surfaces (faces) of the top and bottom mold halves, an advantage is offered in that not only the positioning accuracy required between the mold and the sheet can be lowered, but also the cleaning resin and chemicals can be infiltrated at a portion where the sheet is sandwiched between the main surfaces of the top and bottom mold halves, thereby permitting the mold to be cleaned.
In the technique, however, when the cleaning resin is filled in the cavity, the sheet-shaped member is vertically moved (lifted) within the cavity. This causes a phenomenon wherein the sheet-shaped member becomes resistant to the flow of the cleaning resin, not allowing the cleaning resin to be run to all corners of the cavity.
Eventually, dirt is left at the corners of the cavity, with the attendant problem that cleaning within the cavity becomes unsatisfactory.
It should be noted that the faces of the mold have recesses, such as a flow cavity (capable of escaping air or a molding resin to prevent air from being entrained from a gate and ensure well-balanced filling of the molding resin in the cavity) and an air vent, in communication therewith formed at corners of the outer periphery of the cavity.
However, where the molding is subjected to cleaning with use of a cleaning sheet for a mold, the cleaning resin is unlikely to creep in the corners of the cavity and vicinities thereof. As a result, the cleaning resin does not enter into the flow cavity and air vent, and thus the flow cavity and air vent cannot be cleaned. This arises the problem of shortage of a molding resin being filled into the cavity upon molding of a product after cleaning.
Further, if the molding resin is unlikely to creep in the corners of the cavity and vicinities thereof, the cleaning resin is not stuck at portions of the molding cleaning sheet corresponding to the cavity corners and vicinities thereof. This presents the problem that after completion of the cleaning, when the cleaning resin is removed from the faces of the mold along with the mold cleaning sheet, the cleaning resin is partly left, which takes much time in removing the mold cleaning sheet and the cleaning resin from the faces.
In the faces of a mold for molding of SOP (small outline package) having an elongated portion to be molded and QFN (quad flat non-leaded package) using a matrix frame, where the distance between the outer edge of a cavity and the margin of the face is relatively small (e.g. where the distance is 10 mm or below), the cleaning resin leaked from the cavity runs out of the mold cleaning sheet upon cleaning of the mold and attaches to over the side surfaces contiguous to the faces of the mold.
In this case, it takes much time to remove the cleaning resin from the side surfaces of the mold, with the attendant problem that the efficiency of the cleaning work lowers.
An object of the invention is to provide a mold cleaning sheet which is able to improve the cleaning effect of a mold and also improve productivity through the reduction in time of a cleaning work and also a method of producing semiconductor devices using the mold cleaning sheet.
The above and other objects and novel features of the invention will become apparent from the description of the present specification and the attached drawings.